The present invention relates to self-piercing rivets used to join together metallic sheets, and more particularly to a self-piercing rivet having features which inhibit rotation of the rivet with respect to the metallic sheets joined thereby.
The joining of metallic sheets can be accomplished by various mechanical modalities, as for example, threaded fasteners, rivets and welding. The modality used depends upon the application. One type of rivet having an excellent ability to join together a stack of metallic sheets is a self-piercing rivet.
Various aspects of a prior art self-piercing rivet are shown at FIGS. 1 through 4. A self-piercing rivet 10 has a hollow shell 12 of cylindrical shape which is closed at one end by an overhanging head 14 and which has at the opposite end a point 16. A ram 18 and opposing die 20 are used to drive the self-piercing rivet 10, point 16 first, into two or more sheets 22, typically a metal, such as for example aluminum. The ram 18 has a convex contour 24 and the die has a concave contour 26, such that after stroking of the ram, the hollow shell 12 pierces the sheets 22 with a deformation D defined by the concave contour 26 and wherein the head 14 is countersunk by the convex contour 24. The deformation D involves a bending of the hollow shell 12 outwardly so as to lock the self-piercing rivet 10 in position with respect to the sheets 22 such as to prevent its removal from the sheets and, as a result, any possible disjoinder of the sheets.
It will be noticed that the deformation D, while preventing the self-piercing rivet and the sheets 22 from releasing from one another, involves a circular symmetry about the shell axis A of the hollow shell 12. Because of this a circular symmetry, there is the possibility that, in spite of rivet clamping and the resultant high frictional forces which are present, the sheets 22 may, over time, rotatively wander relative to each other and one or more with respect to the self-piercing rivet. The possibility of rotational wandering of the sheets relative to each other can be undesirable in a number of self-piercing rivet applications.
Accordingly, what is needed in the art is some way to prevent rotational wandering of sheets joined by a self-piercing rivet.
The present invention is a self-piercing rivet wherein the hollow shell has an external axial asymmetry wherein the asymmetry is with respect to the centerline axis of the hollow shell (or simply put, the shell axis). Once installed in a stack of sheets, the asymmetric self-piercing rivet prevents relative sheet rotation of the sheets relative to the shell axis because of rotational interference caused by the external axial asymmetry of the hollow shell.
A preferred external axial asymmetry of the hollow shell is provided by the exterior surface of the hollow shell having irregular surface features which are oriented generally parallel to the shell axis. The irregular surface features may be either protruding surface features, such as for example ribs, or intruding surface structures, such as for example flutes (i.e., grooves or slots). In operation, as the asymmetric self-piercing rivet is driven into a plurality of sheets (composed of for example metal or metal composite) to be joined, the irregular surface features interact with the sheets such that after joinder, the sheets are prevented from relative rotation because of an interference fit between the sheets and the irregular surface features.
The external axial asymmetry can be provided alternatively, or in addition to the aforementioned surface irregularity features, by any geometrical configuration of the hollow shell which is non-circular with respect to the shell axis, as for example an oval or square geometry with respect to the shell axis of rotation of the hollow shell.
Accordingly, it is an object of the present invention to provide a self-piercing rivet having an external axial asymmetry which serves to prevent relative rotation of sheets joined thereby.
This and additional objects, features and advantages of the present invention will become clearer from the following specification of a preferred embodiment.